What is the X in X-mas?
mid-14c., from Anglo-French popler, from Old French poplier (13c., Modern French peulplier), from Latin populus "poplar" (with a long "o;" not the same word that produced popular), of unknown origin, possibly from a PIE tree-name root *p(y)el- (cf. Greek pelea "elm"). Italian pioppo, Spanish chopo, German pappel, Old Church Slavonic topoli all are from Latin.
Heb. libneh, "white", (Gen. 30:37; Hos. 4:13), in all probability the storax tree (Styrax officinalis) or white poplar, distinguished by its white blossoms and pale leaves. It is common in the Anti-Libanus. Other species of the poplar are found in Palestine, such as the white poplar (P. alba) of our own country, the black poplar (P. nigra), and the aspen (P. tremula). (See WILLOW.)